The present invention relates to a method for the preparation of a sex pheromone dispenser used in the elimination of pest insects by means of sustained release of the sex pheromone compound of the insects which, when contained in an extremely low concentration in the atmospheric air, can disturb the intercommunication between different sexes of the insects so as to disrupt mating of them. More particularly, the invention relates to a method for the preparation of a sex pheromone dispenser capable of sustainedly releasing the pheromone compound at a uniform rate even at a relatively low temperature.
As one of the methods for the elimination of pest insects from the agricultural fields, the method by the utilization of a sex pheromone compound of the specific insect species is highlighted in recent years, in which the synthetically prepared sex pheromone compound, referred to simply as pheromone hereinafter, is released to the atmosphere of the field and contained in the air so that the males of the insect are disturbed from recognition of the females to mate which releases the same pheromone compound to attract the male. This method of mating disruption is highlighted because the method is absolutely free from the problem of environmental pollution which is unavoidable in the use of a large quantity of insecticide compounds having toxicity not only against the pests but also against human body.
Since development of pest insects is repeated several times during the seasons beginning with spring and ending in autumn in the year, the method of pest elimination by use of a pheromone is usually undertaken by distributing a large number of pheromone dispensers over the field during the period in which the adult insects of the first generation develop so as to release the pheromone compound sustainedly through the summer to autumn.
Various attempts and proposals have been heretofore made in order to improve such pheromone dispensers relative to the sustained releasability of the pheromone. Following are some of the examples of those improved pheromone dispensers as proposed.
U.S. Pat. No. 4,017,030 proposes a pheromone dispenser of a type in which the pheromone compound is contained in a capillary tube closed at one end and opening at the other end and the pheromone is sustainedly released into the atmosphere out of the open end. The pheromone dispensers of this type, however, are not suitable for practical use because the amount of the pheromone compound contained in a single dispenser tube is usually very small so that the dispenser has a limited life for sustainedly releasing the pheromone.
Pheromone dispensers of a second type include those proposed in U.S. Pat. No. 4,160,335 in which a carrier layer of a polymer admixed with the pheromone is laminated with a layer of a polymer as a releasing-controlling layer to form a laminated body and those proposed in Japanese Pat. Kokai No. 59-13701 and No. 59-59734 and U.S. Pat. No. 4,445,641 in which a porous body impregnated with a pheromone is coated with a coating layer of polyethylene and the like to control the permeating rate of the pheromone therethrough. These pheromone dispensers have a defect in common that a considerably large portion of the pheromone initially contained in the dispenser remains as unreleased even after expiration of the life of the dispenser.
As the pheromone dispensers of a third type, U.S. Pat. Nos. 2,800,457, No. 2,800,458 and No. 3,577,515 propose microcapsules in which a pheromone compound is encapsulated. These pheromone-containing microcapsules, however, are far from practicality because the cost for the preparation of microcapsules is high and a considerably large portion of the expensive pheromone compound is lost in the course of the preparation of the microcapsules in addition to the defect that the rate of pheromone release is usually too large due to the large surface area of microcapsules so that the serviceable life of the pheromone dispensers of the microcapsule type cannot be long enough.
The pheromone dispensers of a fourth type are proposed in Japanese Patent Kokai No. 62-195303 and elsewhere in which a pheromone is contained in a container in the form of a capillary tube or ampule made from a uniform simple layer of a polymer film. In particular, the polymeric film material may have a specific equilibrium swelling ratio with the pheromone compound.
The container of the fourth-type pheromone dispensers is usually made from a polyolefin film of a relatively large thickness which ensures good sustained releasability of the pheromone to impart the dispenser with a long serviceable life. On the other hand, such a pheromone container causes following problems because the pheromone must permeate through the wall of the container so that the rate of pheromone release to the atmosphere largely depends on the temperature.
When the pheromone dispenser is to be used throughout the year, namely, a sufficiently high rate of release cannot be obtained under a climatic condition of low temperatures as in early spring, when most of the first-generation pest insects develop. When the barrier performance of the polymer film is decreased by using a polymer film of a decreased thickness or by using a polyolefin of a low crystallinity in order to ensure high releasability of the pheromone, on the other hand, a problem is caused that not only the rate of pheromone release is too high when the ambient temperature is high as in summer but also the liquid pheromone infiltrates the walls of the dispenser so as to increase the unavailable portion of the pheromone contained in the dispenser. When the pheromone dispenser is prepared such that the rate of pheromone release is appropriate at high temperatures of summer, on the other hand, the rate of pheromone release would be too low at low temperatures or, in particular, at the initial stage of use so that the object of pest elimination cannot be fully achieved.
When the pheromone compound is an aldehyde, acetate, ketone, hydrocarbon or alcohol of 10 to 22 carbon atoms in a molecule having aliphatic unsaturation, as is the case of most of the pheromone compounds of the insects belonging to the order of Lepidoptera, the pheromone compound infiltrating the walls of the dispenser and retained there is subject to denaturation by the reaction of polymerization, oxidation, bond cleavage and the like. This problem can of course be solved by enhancing the barrier performance of the dispenser walls though at the sacrifice of the pheromone releasability not to ensure sufficiently high rate of pheromone release at low temperatures in early spring.